Many spacecraft, such as Earth-orbiting communication satellites, require a particular attitude relative to the Earth. Maneuvers to acquire the Earth should take minimal time, since loss of Earth lock often means that the spacecraft is not performing mission objectives. A related concern is that the spacecraft may be in unsafe thermal attitudes for an unacceptable duration. Many maneuvers to acquire the Earth currently require torques of a magnitude that can only be supplied by thrusters, using limited fuel thereby potentially shortening the effective life of the spacecraft mission.
A typical Earth acquisition method is described in U.S. Pat. No. 5,080,307 titled "Spacecraft Earth-Pointing Attitude Acquisition Method" to Smay, Yocum and Hummel. The method starts with the spacecraft roll axis initially pointing towards the Sun. An Earth sensor is along the yaw axis. Hence, the first maneuver is a slew about the pitch axis. The slew is stopped when the Earth sensor boresight crosses a Sun centered Earth cone. This cone is formed by taking a line from the spacecraft to the Earth (Earthline) and sweeping it around an axis formed by a line between the spacecraft and the Sun (Sunline). The cone is determined from ground-based calculations and transmitted to the spacecraft. In the next step, the spacecraft performs a coning maneuver about the Sunline, moving the boresight of the Earth sensor along the Sun-centered Earth cone. Under normal conditions, the maneuver halts when the Earth sensor fully detects the Earth.
Several difficulties arise with the maneuver just described and other similar methods. First, the spacecraft must be initially pointing towards the Sun.
Second, the coning maneuver about the Sunline does not account for time-varying Sun-Earth geometry. Such variations are important in low or medium Earth orbits which have orbit times that may be a fraction of one day. Not compensating for changing Sun-Earth geometry limits the time in which the coning maneuver can be accomplished, thereby requiring large coning rates to ensure that the Earth does not move out of the sensor swept field-of-view before the search is complete. Typically, thrusters are required to realize the large coning rates, expending limited fuel.
Third, the method does not provide the capability to orient the spacecraft in an optimal thermal attitude.
Fourth, ground-based calculations during the maneuver are required.